This invention relates to demodulation of a Quadrature Amplitude Modulation (QAM) signal. In particular, it relates to the derivation of a local oscillator output which is at the frequency of the suppressed carrier of the QAM signal and which is phase aligned with respect to the QAM signal for properly demodulating it.
In a basic digital transmission system, the baseband signal has one of two values, designated logic 0 or logic 1. In the receiver of a digital radio for such a system, the technical problems concern identifying which of these two states is being received in a given bit interval. The problems of accurately performing this identification in the presence of various forms of noise and distortion are well known.
In a QAM system, there are more states. For example, in a 16 QAM system, the problem becomes one of identifying which of sixteen states has been transmitted. These are not sixteen different signal amplitude levels, but sixteen different combinations of amplitudes and phases in a vector space representation of the signals.
The QAM signal is a suppressed carrier signal. In order to demodulate it, it is necessary to somehow derive a local oscillator voltage having the precise frequency of the suppressed carrier of the received QAM signal and having the exact phase alignment with the QAM signal as did the carrier. It should be apparent from the large number of states transmitted and the complexity of the resulting QAM signal waveform, that deriving the local oscillator signal from the QAM signal waveform represents a considerable technical challenge. Of course, the problem is compounded since the received QAM signal is distorted by the transmission process.